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From: beard@ux1.lbl.gov (Patrick C Beard)
Newsgroups: alt.bbs,comp.sys.mac.programmer,comp.sys.mac,comp.sources.wanted
Subject: Re: Wanted: CRC-16 Routines in Pascal (C table driven version)
Keywords: crc xmodem bbs help wanted pascal checksum table-driven
Message-ID: <1235@helios.ee.lbl.gov>
Date: 8 Nov 88 07:36:25 GMT
References: <1971@uokmax.UUCP> <6672@venera.isi.edu>
Sender: usenet@helios.ee.lbl.gov
Reply-To: beard@ux1.lbl.gov (Patrick C Beard)
Distribution: usa
Organization: Lawrence Berkeley Laboratory, Berkeley
Lines: 81

In article <6672@venera.isi.edu> dick@venera.isi.edu.UUCP (Richard Gillmann) writes:
>
>Here is a version of the CRC calculation as used by XMODEM, YMODEM, etc.
>It is written in Microsoft Pascal.  The umulok routine is a library call
>that does an unsigned multiply and allows overflow.  It's used to
>simulate a shift and check of the carry bit.  Of course, this could be
>much speeded up by writing in assembler or using tables.
>
>{Computes a 16-bit circular redundancy check.  Initialize your variable for
>  crc_value to 0, then call this routine for each byte.} 
>procedure crc_16(c : char; var crc_value : word);
>begin
>  crc_value := crc_value xor (wrd(c)*256);
>  for var i := 1 to 8 do
>    if not umulok(crc_value,2,crc_value) then {hi bit was a 1}
>      crc_value := crc_value xor 16#1021; {X^15+X^12+X^5+1}
>end {crc_16};

Here is a table driven version (in C, sorry).  crc should be set to
zero before using COMPUTE_CRC.

/*
	crc.c -- routine for calculating a CRC-16.
	Put together by Patrick Beard, from Xmodem.3.6 distribution, by

 */

/*   CRC-16 constants.  From Usenet contribution by Mark G. Mendel,
     Network Systems Corp.  (ihnp4!umn-cs!hyper!mark)
*/

    /* the CRC polynomial. */
#define P   0x1021
    /* number of bits in CRC */
#define W   16
    /* the number of bits per char */
#define B   8

/*   CRC-16 constant array...
     from Usenet contribution by Mark G. Mendel, Network Systems Corp.
     (ihnp4!umn-cs!hyper!mark)
*/

/* crctab as calculated by initcrctab() */
unsigned short crctab[1<<B] = {
    0x0000,  0x1021,  0x2042,  0x3063,  0x4084,  0x50a5,  0x60c6,  0x70e7,
    0x8108,  0x9129,  0xa14a,  0xb16b,  0xc18c,  0xd1ad,  0xe1ce,  0xf1ef,
    0x1231,  0x0210,  0x3273,  0x2252,  0x52b5,  0x4294,  0x72f7,  0x62d6,
    0x9339,  0x8318,  0xb37b,  0xa35a,  0xd3bd,  0xc39c,  0xf3ff,  0xe3de,
    0x2462,  0x3443,  0x0420,  0x1401,  0x64e6,  0x74c7,  0x44a4,  0x5485,
    0xa56a,  0xb54b,  0x8528,  0x9509,  0xe5ee,  0xf5cf,  0xc5ac,  0xd58d,
    0x3653,  0x2672,  0x1611,  0x0630,  0x76d7,  0x66f6,  0x5695,  0x46b4,
    0xb75b,  0xa77a,  0x9719,  0x8738,  0xf7df,  0xe7fe,  0xd79d,  0xc7bc,
    0x48c4,  0x58e5,  0x6886,  0x78a7,  0x0840,  0x1861,  0x2802,  0x3823,
    0xc9cc,  0xd9ed,  0xe98e,  0xf9af,  0x8948,  0x9969,  0xa90a,  0xb92b,
    0x5af5,  0x4ad4,  0x7ab7,  0x6a96,  0x1a71,  0x0a50,  0x3a33,  0x2a12,
    0xdbfd,  0xcbdc,  0xfbbf,  0xeb9e,  0x9b79,  0x8b58,  0xbb3b,  0xab1a,
    0x6ca6,  0x7c87,  0x4ce4,  0x5cc5,  0x2c22,  0x3c03,  0x0c60,  0x1c41,
    0xedae,  0xfd8f,  0xcdec,  0xddcd,  0xad2a,  0xbd0b,  0x8d68,  0x9d49,
    0x7e97,  0x6eb6,  0x5ed5,  0x4ef4,  0x3e13,  0x2e32,  0x1e51,  0x0e70,
    0xff9f,  0xefbe,  0xdfdd,  0xcffc,  0xbf1b,  0xaf3a,  0x9f59,  0x8f78,
    0x9188,  0x81a9,  0xb1ca,  0xa1eb,  0xd10c,  0xc12d,  0xf14e,  0xe16f,
    0x1080,  0x00a1,  0x30c2,  0x20e3,  0x5004,  0x4025,  0x7046,  0x6067,
    0x83b9,  0x9398,  0xa3fb,  0xb3da,  0xc33d,  0xd31c,  0xe37f,  0xf35e,
    0x02b1,  0x1290,  0x22f3,  0x32d2,  0x4235,  0x5214,  0x6277,  0x7256,
    0xb5ea,  0xa5cb,  0x95a8,  0x8589,  0xf56e,  0xe54f,  0xd52c,  0xc50d,
    0x34e2,  0x24c3,  0x14a0,  0x0481,  0x7466,  0x6447,  0x5424,  0x4405,
    0xa7db,  0xb7fa,  0x8799,  0x97b8,  0xe75f,  0xf77e,  0xc71d,  0xd73c,
    0x26d3,  0x36f2,  0x0691,  0x16b0,  0x6657,  0x7676,  0x4615,  0x5634,
    0xd94c,  0xc96d,  0xf90e,  0xe92f,  0x99c8,  0x89e9,  0xb98a,  0xa9ab,
    0x5844,  0x4865,  0x7806,  0x6827,  0x18c0,  0x08e1,  0x3882,  0x28a3,
    0xcb7d,  0xdb5c,  0xeb3f,  0xfb1e,  0x8bf9,  0x9bd8,  0xabbb,  0xbb9a,
    0x4a75,  0x5a54,  0x6a37,  0x7a16,  0x0af1,  0x1ad0,  0x2ab3,  0x3a92,
    0xfd2e,  0xed0f,  0xdd6c,  0xcd4d,  0xbdaa,  0xad8b,  0x9de8,  0x8dc9,
    0x7c26,  0x6c07,  0x5c64,  0x4c45,  0x3ca2,  0x2c83,  0x1ce0,  0x0cc1,
    0xef1f,  0xff3e,  0xcf5d,  0xdf7c,  0xaf9b,  0xbfba,  0x8fd9,  0x9ff8,
    0x6e17,  0x7e36,  0x4e55,  0x5e74,  0x2e93,  0x3eb2,  0x0ed1,  0x1ef0
    };

#define COMPUTE_CRC(crc,byte) \
	crc = (crc<<B) ^ crctab[(crc>>(W-B)) ^ byte];